Police officers who visited the "virgin killer" Elliot Rodger just weeks before he went on a deadly rampage had been warned about disturbing videos he posted online - but failed to check them, US police have disclosed.

The British-born gunman was questioned by six officers at his home three weeks before his killing spree after his mother and friend grew alarmed at his behaviour and alerted an emergency mental health line.

Rodger, 22, had been stockpiling weapons in his apartment and plotting his rampage through Isla Vista, but he put on a "timid" appearance to officers and told them he made the videos as a means of expressing himself.

Police stayed eight minutes, did not search his home and did not watch the videos, which would have given a chilling glimpse into the murderer's killing plans.

In a statement, the Santa Barbara County Sheriff's Office said: " Deputies found Rodger to be shy, timid and polite. When questioned by the deputies about reported disturbing videos he had posted online, Rodger told them he was having trouble fitting in socially in Isla Vista and the videos were merely a way of expressing himself.

"Based upon the information available to them at the time, sheriff's deputies concluded that Rodger was not an immediate threat to himself or others, and that they did not have cause to place him on an involuntary mental health hold, or to enter or search his residence. Therefore, they did not view the videos or conduct a weapons check on Rodger."

An officer called Rodger's mother at the scene and passed the phone over to the 22-year-old, who reassured her he was "fine" and would speak to her later.

He later described just how close the police came to unravelling his plans in his rambling 137-page killing manifesto which he sent to his parents and therapists on the day of the attacks.

In it he said: "The police interrogated me outside for a few minutes, asking me if I had suicidal thoughts. I tactfully told them that it was all a misunderstanding, and they finally left.

"If they had demanded to search my room... That would have ended everything. For a few horrible seconds I thought it was all over. When they left, the biggest wave of relief swept over me."

Just three weeks later, on May 23, he embarked on a killing spree that left six dead and 13 injured in a day of violence he dubbed "the day of retribution".

He stabbed to death his three male flatmates before getting in to his car and speeding through the town shooting at horrified bystanders. He fatally shot two women outside a sorority house before eventually killing himself with a gunshot to the head.

The sheriff's office also gave fresh details about the timeline leading up to the slaughter.

They said Rodger uploaded a YouTube video detailing his killing plans at 9:17pm on the day of the shootings, and one minute later emailed his 137-page "manifesto" to his parents and therapist.

The first gunshots were reported at 9:27 pm and approximately eight minutes later the slaughter was over.

But it was another half an hour before the therapist discovered the manifesto, and she called police at around 10.11pm.

Officers then contacted Rodger's mother and first viewed the rambling manifesto and "Retribution" video at about 10.26pm - around an hour after the killings.

In the video, he calmly says: " Tomorrow is the day of retribution, the day in which I will have my revenge against humanity. Against all of you.

"For the last eight years of my life, ever since I hit puberty, I've been forced to endure an existence of loneliness, rejection and unfulfilled desires. All because girls have never been attracted to me.

"Girls gave their affection and sex and love to other men but never to me.

"I'm 22 years old and still a virgin. I've never even kissed a girl."

Police said the investigation is "one of the most complex investigations in the history of the Santa Barbara County Sheriff's Office" and said they are trawling through reams of material and carrying out numerous interviews.

They added: "Due to the fact that this investigation is ongoing, no further information related to the case will be released at this time."